This invention relates to the destruction of spent potliners derived from the metal processing industry.
During the production of aluminum, reinforced steel pots (cells) lined with brick hold the cryolite bath which is required for the electrolytic reduction of alumina. After usage, the linings of these pots degrade to the point that they no longer function properly. The spent potliners typically have been landfilled. The aluminum industry generates spent potliners at a rate of roughly 2% to 4% by weight of the aluminum produced. The production of spent potliner in the U.S. is approximately 120,000 tons per year.
Spent potliner has a following approximate analysis with some variation:
______________________________________ Component wt % ______________________________________ carbon 60.0 sodium oxide 14.0 aluminum oxide 11.0 iron oxide 0.4 silicon dioxide 0.5 calcium oxide 1.25 fluorine 15.0 cyanide 0.2 ammonia 0.005 ______________________________________
The presence of leachable cyanide, fluoride and trace metals caused the U.S. Environmental Protection Agency to classify spent potliner as a listed hazardous waste (K088). The EPA then promulgated rules which impose treatment standards and restrict the land disposal of K088 spent potliner wastes (see 40 CFR part 268, Apr. 8, 1996). At that time, EPA believed the rotary kiln at the Reynolds facility in Gum Springs, Ark. would provide sufficient treatment capacity for the entire U.S. annual burden of spent potliner. Subsequent events discussed below have demonstrated that the Reynolds rotary kiln did not adequately treat the spent potliner as the residue from the kilns exhibited unacceptable leaching profiles, especially for cyanide and fluoride. EPA delayed the imposition of the treatment standards for spent potliner.
Technology has been proposed for the destruction of spent potliners from the aluminum manufacturing industry. The primary method proposed for the destruction of the spent potliners is exemplified by the rotary kilns at the Reynolds facility in Gum Springs, Ark. and U.S. Pat. No. 5,164,171 which discloses the detoxification of spent potliner material ("SPL") by the crushing and sizing of spent potliner materials along with the addition of roughly equal portions of limestone and sand as both a fluxing agent and a reactant, followed by the feeding of the mixture to a rotary kiln at a temperature of between 1200-1700 degrees Fahrenheit for the thermal destruction of the cyanides, polyaromatic hydrocarbons, and other organic materials found in the spent potliners. The combined feeds are heated in the kiln, the organic materials are destroyed via oxidation or combustion with oxygen, and the inorganic materials are removed from the kiln as a residue. This residue is claimed to be non-leachable by EPA standards under the TCLP requirement. In fact, however, the total destruction of the organic materials is not achieved. It has been found that leachate from the residue materials contain significantly higher quantities of the cyanide than had been predicted. In addition, the leachate contains large quantities of fluoride, levels well above national standards. As a result this technology, while currently approved by EPA, is not acceptable to the industry as a whole. The use of oxygen and steam to gasify the carbon content of the SPL, producing a valuable synthesis gas, is not disclosed. Alternative technologies have been sought.
An alternative technology has been proposed by Vortec Corp. in U.S. Pat. No. 5,558,690. Vortec has created an above ground oxidation and vitrification process for the remediation of spent potliner material. This system vitrifies the waste materials and presumably totally encapsulates the inorganic material while at the same time destroys any organic material found in the spent potliners material. At the heart of the process is the Vortec cyclone melting system, which consists of a counter-rotating vortex combustor and a cyclone melter. For proper vitrification to take place, the spent potliner material may be mixed with glass making ingredients such as silicon dioxide and limestone. This mixture, along with propane fuel and essentially pure oxygen, is introduced to the counter-rotating vortex combustor. Temperatures of up to three thousand degrees Fahrenheit destroy the organic compounds and melt the inorganic materials. This slag mixture which is formed then is thrown to the cyclone melter wall. The molten glass is quenched in water and removed for storage. The average temperature of the materials leaving the combustion chamber is typically between 2200 and 2700 degrees Fahrenheit prior to being quenched in the water. Typically in the Vortec process, a relatively large excess of oxygen is utilized resulting in downstream handling problems and increased costs. An additional fuel such as propane or natural gas is utilized in this technology, thus increasing the cost significantly. The products of combustion, carbon dioxide and water, are released as stack emissions to the environment. Although this technology claims to be strictly recycling, the only material actually recycled is the inorganic component of the spent potliner material. This reclaimed material is a frit which is claimed to be recyclable and utilized with in the aluminum industry. Some of the fluorine is recovered overhead as hydrogen fluoride which then is recycled back to the aluminum industry as aqueous hydrofluoric acid. The carbon contained within the spent potliners is not recoverable and is released to the atmosphere as carbon dioxide rather than being converted to a valuable synthesis gas via gasification.
Another technology, from Ausmelt Ltd. of Australia, has been proposed to be able to handle spent potliner material. In this process, spent potliner and flux are melted at 2000-2200 degrees Fahrenheit while oxygen is blown into the melt through a submerged lance to fuel the combustion process. Oxygen is used rather than air to increase concentration of hydrogen fluoride in the off gas. Presumably the use of pure oxygen to combust all of the carbon contained within the material is an improvement in that it is easier to treat the hydrofluoric acid that comes out over head. In this process, cyanide is destroyed and all of the carbon present in the spent potliners material is oxidized/combusted to form carbon dioxide. All inorganic components excluding the overhead hydrogen fluoride are recovered in an inert slag. Presumably this slag may be used as road building aggregates and for other purposes. The overhead hydrogen fluoride can be used to produce aluminum fluoride which is an additive in the aluminum smelting industry. The carbon contained within the spent potliners is not recoverable and is released to the atmosphere as carbon dioxide.
U.S. Pat. No. 4,993,323 teaches the fluidized bed combustion of SPL at 800-1000 degrees Centigrade. A mixture of SPL, limestone and lignite is granulated and fully combusted to form carbon dioxide and a free-flowing ash. Production of a synthesis gas, hydrogen fluoride or an environmentally benign vitreous frit via gasification processes are not disclosed in this patent.
U.S. Pat. No. 4,735,784, assigned to Morrison-Knudsen Company, Inc., discloses a method for treating SPL which involves adding silica to the SPL to form a mixture, heating this mixture to 1,000-1,700 degrees Centigrade to form a slag in the presence of sufficient water or steam to volatilize the fluoride containing contaminants in the form of hydrogen fluoride. The remaining slag is cooled to form an insoluble silicate glass with any remaining fluoride in an immobile state. Not disclosed is the production of a valuable Synthesis Gas via gasification.
U.S. Pat. No. 5,222,448, assigned to Columbia Ventures Corporation, discloses a method for treating SPL in which the SPL is exposed to the high heat of a plasma torch as an energy source. Carbon from the SPL is evolved as a carbon oxide or hydrocarbon, mostly as carbon dioxide. Fluoride compounds are either melted or evolved as hydrogen fluoride and all inorganic material is melted to form a slag. The gasification of the SPL with oxygen and steam to form valuable Synthesis Gas and hydrogen fluoride is not disclosed. It was estimated that the power consumption to treat 200 tons/day of SPL by this plasma torch technology is between 3.3-6.6 megawatt hours, continuously. On the other hand, the present invention would result in a net energy production of approximately 14.7 megawatt hours continuously, a significant energy and cost savings not realized in U.S. Pat. No. 5,222,448.
The present inventors are associated with the licensee of U.S. Pat. Nos. 4,869,741, 4,950,309, 5,074,890 and 5,245,113, which are discussed below.
U.S. Pat. No. 4,869,741 entitled "Process for the thermal decomposition of toxic refractory inorganic substances" relates to the destruction of toxic organic refractory substances including halogenated organic biocides, organophosphates, and waste streams from their production. The toxic substances are fed into a refractory lined reaction chamber (a long vertical tube lined with refractory material) along with oxygen and steam in order to convert the toxics into gaseous reaction products comprising carbon monoxide and hydrogen. The reaction takes place at a temperature of 2500 to 3200 degrees Fahrenheit and the reactor is packed with an incandescent solid refractory inorganic oxide such as zirconia or aluminum.
U.S. Pat. No. 4,950,309 entitled "Process for the conversion of toxic organic substances to useful products" discloses a method for destroying toxic refractory substances including sewage sludge, hospital waste, halogenated hydrocarbons, and organophosphates by contacting the toxics with steam or water, oxygen, and incandescent solid carbon such as coke. The toxics, steam and oxygen are initially contacted in a first reaction zone at 2500 to 3200 degrees Fahrenheit to form a reducing gas stream which is then reacted with the carbon in the second reaction zone to recover a product gas comprising carbon monoxide and hydrogen free from organic halides,
U.S. Pat. No. 5,074,890 entitled "Process for the thermal decomposition of toxic refractory organic substances" is similar to the above, and discloses the destruction of toxic refractory substances including dioxins, polyhalogenated biphenyls, halogenated organic biocides and organophosphates. The toxics are initially reacted with steam and oxygen for five to five hundred milliseconds in a refractory lined reaction chamber. The resulting gaseous reaction products then are contacted with incandescent carbon solids contained in a bed at 2500 to 3200 degrees Fahrenheit. The carbonaceous solid is fed downward through a moving bed, and is thereby gasified, in addition to reacting with the gaseous reaction products.
U.S. Pat. No. 5,245,113 entitled "Decontamination of PCB contaminated solids" discloses a method for the detoxification of electrical hardware contaminated with polychlorinated biphenyls (PCB's). The hardware is first contacted with the carrier reducing gas comprising hydrogen and carbon monoxide at a temperature of 750-950 degrees Fahrenheit in a furnace to thereby vaporize the PCBs. The reducing gas is then fed into a reaction zone where the PCBs are destroyed by the partial oxidation of a carbonaceous fuel with oxygen and steam at a temperature of 2500 to 3500 degrees Fahrenheit. A portion of the reducing gas can be sent back to the furnace to vaporize more PCBs. This and all other patents mentioned in this specification are incorporated herein by reference.
The basic technology described in the above four patents, called Slagging Gasification Injection (SGI) technology, is a modification of proven gasification technologies which evolved over a hundred years and was first employed in the early 1960s as an independent technology by the U.S. Department of Energy and developed separately over the period 1974 to 1991 by British Gas. The oxygen blown, fixed bed, slagging gasifier that is the core of SGI technology is similar to, but simpler in operation than, the blast furnace of the steel industry, which has a history of reliable operation of over a century. The high temperatures achievable afford total destruction of the most refractory organic (such as polychlorinated biphenyls and dioxins) at very high throughput rates. The gasifier, with its bed of incandescent carbon, readily accepts wide variations in composition of the organic and inorganic waste streams, and it operates successfully at high or low throughput rates, thus increasing its versatility.
An SGI unit is shown schematically in FIG. 1 and described briefly below. Very simply, the gasifier is a long, vertical tube filled with a constantly descending bed of carbonaceous material. In the patents described above this is generally coke, although coal may be utilized. Additional patents describe the feed of municipal solid waste or sewage sludge solids being fed to the top of the gasification unit. Although individual pieces of coke or coal constantly descend, and are consumed, during the SGI operation, the column of material is kept at a fixed height via a system of feed lock hoppers, locked valves and level gauges. The unit is therefore called a "fixed bed" slagging gasifier. At the bottom of the unit is a hearth with or without a grate to support the carbon substrate bed. At the hearth are injection ports (tuyeres, burners) for the introduction of oxygen, steam and the liquid or pumpable waste materials to be destroyed. The SGI unit is run at a pressure of approximately 350 psig, although higher pressures to attain correspondingly higher throughputs are possible. The following zones exist within the SGI unit once the unit is in operation.
The oxidation zone is present at the point the tuyeres introduce the oxygen, steam and waste streams. Predominantly traditional oxidation or combustion takes place in this zone. Carbon and hydrogen bearing materials (the industrial waste materials and/or coke) are converted into carbon dioxide and water via oxidation or combustion with the oxygen. This zone is similar to an incinerator except for the fact that the unit utilizes pure oxygen rather than the air used by the commercial incinerators. This results in a significantly higher temperature than is possible in a conventional air blown incinerator. The maximum temperatures in the zone are expected to be greater than 2800 degrees Fahrenheit, with actual, in operation temperatures of 3000 to 3600 degrees Fahrenheit. By contrast, conventional incinerators are believed to operate in a maximum temperature less than 2600 degrees Fahrenheit and normal operating conditions are thought to be in the 2100 to 2400 degrees Fahrenheit range. This is a significant difference in operating temperature and will result in exceptional destruction efficiency as well as other benefits for SGI technology. An added benefit is that the use of oxygen rather than air results in a major volume reduction for the SGI product gas as compared to the stack emissions of the traditional incinerator design. Substantial savings in downstream gas purification are realized as a result of the low SGI product gas volume.
Immediately above the short oxidation zone and the point of introduction of the oxygen, is the reduction zone. At this point, the oxygen will have been completely consumed. The residual co-injected steam and the water of combustion will be contacted with the incandescent carbon bed to generate hydrogen, carbon monoxide and some methane and carbon dioxide. This atmosphere coupled with the incandescent carbon bed acts as a powerful reducing medium. The temperature of this reduction zone varies from the hearth temperature of about 3600 degrees Fahrenheit to greater than 2400 degrees Fahrenheit at the four foot level. This harsh reducing atmosphere is present in the unit for a substantial length and provides for a long residence time at high temperatures. The total residence time at temperatures in excess of 2400 degrees Fahrenheit is in excess of two seconds, with temperatures in excess of 3000 degrees Fahrenheit being maintained for over one second, thus insuring total destruction of the waste material fed into the SGI unit.
The technology described in the preceding four patents disclose the destruction of toxic wastes that are either liquids or pumpable solids and which are fed into the hearth zone of the SGI unit. Other patents describe the destruction of municipal solid wastes and sewage sludge solids which are briquetted along with coal or coke and fed to the top of the gasification unit as solids. These patents describe only the use of briquetted material such as municipal solid wastes briquetted with coal or coke. They do not describe, nor do they suggest the processing of spent potliner materials in the SGI unit.